While the extant literature of EU politicization has revolved around parties anchored at the margins of the political spectrum, this working paper considers the political and electoral implications of European integration for social-democratic parties, and the EU-related debates within these parties. My extensive study of speeches at the national party conferences of the British Labour Party, the French Parti Socialiste and the German Sozial-demokratische Partei Deutschlands reveals that members and leaders of each party have to contend with a tension between their perceived (country-specific) constraints and opportunities in relation to European integration. Faced with differentiated motivations – on the one hand, the perception of a strong adaptive pressure as regards social-democratic programmes; on the other, strategic responses to European integration designed to maximize electoral scores – leaders and representatives of the party minority tend to opt for different trade-offs. Among all three party organizations, EU-optimistic views tend to predominate while EU-contestation stems mainly from representatives of each party’s internal opposition. Studying party internal debates thus reveals the existence of a contestation of European integration stemming from mainstream parties, carrying the potential of considerable consequences for EU-politicization and European integration.